1. Field of the Invention
The present invention relates generally to systems and methods for controlling spacecraft or satellites, and in particular to a system and method for stepping spacecraft mechanisms to minimize disturbances generated by the stepping mechanism
2. Description of the Related Art
Three-axis stabilized spacecraft or satellites often have several mechanisms that step frequently during their service life. These mechanisms include mechanisms that rotate the antenna reflectors to specific targets, gimbals that scan a payload image frame across specific area of Earth surface, and solar array drivers that step solar arrays to track the Sun. Typically, the stepping function is implemented by stepper motors coupled to the spacecraft component (payloads or solar arrays) via gear-driven transmissions. Stepper motors are desirable because they are relatively simple to control reliable, lightweight and well adapted to continuous use.
However, the use of a stepper motor in spacecraft with highly flexible structural components such as large deployable payload booms, antenna reflectors, and solar arrays may potentially excite some structural modes of these components and generate significant oscillation disturbances in the spacecraft itself. This disturbance can degrade the spacecraft pointing, cause excessive activity of the spacecraft control actuators, and make autonomous spacecraft momentum dumping difficult. The induced oscillation is particularly critical in spacecraft where absolute platform stability is desirable. Vibrations can cause deterioration of any inertia-sensitive operations of a spacecraft.
This disturbance problem can be ameliorated by a number of techniques. One technique is to employ high bandwidth control loops to mitigate the impact of this disturbance to the spacecraft pointing. However, this technique has significant limitations. For many spacecraft, the structural modes that are excited by the stepping mechanisms are outside of the spacecraft control bandwidth. Consequently, these high-bandwidth control loops have only very limited effects on the disturbance. Further extension of the bandwidth of the control loops to include these structural modes will very often result in control loop stability problems. Furthermore, high-bandwidth control also unnecessarily increases actuator operation, which can increase wear and result in excess energy consumption.
A technique for minimizing the solar array drive stepping disturbance is disclosed in U.S. Pat. No. 4,843,294, entitled “Solar Array Stepping to Minimize Array Excitation,” issued Jun. 27, 1989 to Bhat et al, which is hereby incorporated by reference herein. In this reference, mechanical oscillations of a mechanism containing a stepper motor, such as a solar array powered spacecraft, are reduced and minimized by the execution of step movements in pairs of steps. The period between steps is equal to one-half of the period of torsional oscillation of the mechanism. While this method can reduce structural disturbances, it is not very effective when the mechanism has significant backlash and stiction. This is because the backlash and stiction can significantly interrupt the two-step pattern of this method.
There is therefore a need for a system and method for minimizing disturbances in stepper-motor driven mechanisms that are more robust to mechanism backlash and stiction The present invention satisfies that need.